1. Field of the Invention
The present invention relates to a method and device for local contrast enhancement in a video signal processing system, and more particularly to a method and a device for adaptively performing local contrast enhancement by weighing a video signal according to the magnitude of the local contrast of the video signal and by adding the weighted signal to an original signal. The present application is based upon Korean Application No. 49345/1995, which is incorporated herein by reference.
2. Description of the Related Art
Generally, the image quality of a video signal deteriorates because of several factors. For example, low contrast can result in the deterioration of the quality of an image. Gamma correction and histogram equalization are examples of methods for correcting low contrast. Use of these methods can enhance contrast, but while these methods effect an overall contrast enhancement of the video signal, they cannot optically accompany local contrast enhancement.
The prior art method for local contrast enhancement is to lowpass-filter the input video signal and to add the lowpass-filtered value to the enhanced contrast value. The above method is disclosed in U.S. Pat. No. 4,825,297 issued to Fuchsberger. FIG. 1 is a diagram illustrating the construction of a prior art circuit which uses a lowpass filter for acquiring an enhanced local contrast of a video signal. A final output g of FIG. 1 is obtained according to the following expression (1). EQU g=K1(m)+K2(f-m) (1)
wherein, K1 and K2 designate characteristic curves, m designates an output of the lowpass filter, f designates an input of the lowpass filter, and g designates the final output of the lowpass filter.
In the above expression (1), K2 (f-m) has the characteristic as shown in FIG. 2, which is a diagram illustrating the non-linear amplification characteristics curve K2 in a prior art high-pass channel.
In the above U.S. Patent, the local contrast (f-m) is adaptively weighted and the lowpass-filtered signal (m) is added to the local contrast weighted by a factor of K1, thereby outputting an output signal. The reason that the local contrast is adaptively weighted is to prevent deterioration of the image quality and excessive enhancement of the local contrast due to an increased noise level. In other words, if the local contrast is less than an arbitrary value, that is, if A2&lt;f-m&lt;A1 (herein, A1 and A2 are given constants), there is noise in the contrast. In this case, to prevent the noise from being increased, no contrast enhancement is performed. That is, k2(f-m)=0. If the local contrast is greater than the arbitrary value after splitting the local contrast area, in order to reduce an increase of the noise, the contrast should be over-enhanced.
Since the value of the local contrast corresponds to the sum of the lowpass-filtered signal and the adaptively weighted local contrast, there are limits to the application of this technique, as can be seen from the following examples.
(a) When the value of the local contrast is low, the contrast can be generated due to the noise. Thus, the input video signal is outputted unchanged by contrast enhancement. Namely, g=f.
(b) When the value of the local contrast is high, the contrast of the input signal is sufficient. Thus, the input video signal is outputted unchanged by enhancement of the contrast. Namely, g=f.
In the foregoing patent, the above function is not performed because the lowpass-filtered signal constructs the output signal. In the case of (a), the output signal g=m and in the case of (b), g.apprxeq.m. At a result, blurring is caused in the above cases (a) and (b), and it is preferable not to perform the above function.